Method and device to enhance image quality in digital video processing systems using dithering

ABSTRACT

A processing chain for a digital image signal applies a dither pattern, having a first spectrum, to the image signal at a point in the processing chain. A further noise pattern is applied to the image signal during the processing chain. The noise pattern has a second spectrum which is configured such that the combination of the first spectrum and second spectrum results in a more continuous spectrum. Another aspect describes a noise pattern which can be used as an offset dither pattern for digital images, especially before color bit depth reduction. The noise pattern has an array of values which are linearly distributed across a range, with each value in the range occurring an equal number of times.

FIELD OF THE INVENTION

The present invention relates to digital still image or moving imageprocessing systems where dither is applied to a digital image signal.

BACKGROUND TO THE INVENTION

Whenever the bit depth of an image needs to be reduced in a digitalsystem a technique called dithering can be used to represent theoriginal high bit depth image by fewer bits. The idea behind this isthat by introducing small modulations to the image signal, before thetruncation operation, the image signal after truncation to a lower bitdepth will have a correct average value in an area as small as possibleand/or a period as small as possible. A widely used technique is offsetpattern dithering which can be implemented with relatively simplecircuitry and which has predictable and reliable results.

Typically the size of the offset dither pattern is 2×2 or 4×4 pixels andthe optional periodic modulation of these patterns occurs over 2 or 4frames. Larger patterns can introduce too noticeable image artifacts,such as disturbing patterns or interferences with image details. Slowerperiodical modulations can introduce too noticeable flickering of theimage.

Dithering is useful in many cases. Some example scenarios will now bedescribed:

-   -   consider that an image generator renders a high resolution image        with 10-bit precision per colour. The bit depth might need to be        reduced to 8-bits per colour to use a standard transmission        channel or a standard interconnection such as Digital Visual        Interface (DVI) to connect the image generator to a display        device such as a flat screen monitor or a projector. Dithering        will cause the average of the 8-bit per colour output to be        equal to the 10-bits per colour input in each 2×2 area of the        image. Periodic modulation of the truncation process can be        optionally applied to trade off between small spatial artifacts        and temporal flickering.    -   consider that an image is processed or rendered with high bit        depth but needs to be stored in a memory with limited density or        bandwidth. Amongst other techniques, dithering can be applied to        maintain colour information as much as possible while meeting        the density or bandwidth limitation.    -   Digital displays as well as analog displays driven by a digital        to analog converter have a finite bit depth per colour, often 8,        10 or 12 bits per colour. In many cases the digital image        processing prior to the display needs more processing accuracy.        Dithering is applied to resolve as much colour detail as        possible when displaying the image. This is currently performed        on display technologies such as Liquid Crystal Display (LCD),        Liquid Crystal on Silicon (LCOS) and Digital Light Processing        (DLP) based displays and is applicable to many other        technologies.

A practical example of a 2×2 dither pattern is illustrated in FIG. 1c .Consider an image is generated at 10-bits per colour and must be sent toa projector using an 8-bit DVI link. The image generator (IG) has 4times more gradations for each colour than the link supports. That meansa truncation of the 10-bit values must be made prior to transmission viathe 8-bit DVI link. The link only accepts integer 8-bit values in arange from 0 (black) to 255 (white). The IG generates integer values ina range from 0 (black) to 1023 (white) in integer steps. Ignoring therange above 1020, this is practically equivalent to an IG generatingvalues in a range from 0 (black) to 255 (white) in quarter steps.Dithering is the process to truncate these quarter step IG values tointeger 8-bit values in such a way that a small area has the correctaverage value. Optionally the small area average accuracy can beimproved by temporal dithering modulation.

When scanning a video image, a column and line counter is usuallyavailable to the processing. A simple pattern can be constructedelegantly with XOR (exclusive or) equations of some of the leastsignificant bits of both counters. Optionally, the least significantbits of the frame counter can contribute to these equations.

A few typical sets of equations to generate such offset patterns areincluded here as examples. These offset values are then added to thevideo data prior to truncation. A first example of a commonly used 2-bit2×2 spatially fixed dither pattern is shown in FIG. 1c . FIGS. 1a and 1bshow how the pattern of FIG. 1c is obtained. The most significant bitP(1) is obtained by P(1)=column(0) XOR line(0) (FIG. 1a ) and the leastsignificant bit P(0) is obtained by P(0)=column(0) (FIG. 1b ). FIG. 2eshows a 4-bit spatially fixed 4×4 pattern. The most significant bit P(3)is obtained by P(3)=column(0) XOR line(0) (FIG. 2a ), the second mostsignificant bit P(2) is obtained by P(2)=column(0) (FIG. 2b ), thesecond least significant bit P(1) is obtained by P(1)=column(0) XORcolumn(1) XOR line(1) (FIG. 2c ) and the least significant bit P(0) isobtained by P(0)=column(0) XOR line(1) (FIG. 2d ).

A third example is derived from the previous one by extending theequations using the least significant bit of the frame counter totemporally modulate the pattern.

The equations above become:P(3)=column(0) XOR line(0) XOR frame(0)P(2)=column(0) XOR frame(0)P(1)=column(0) XOR column(1) XOR line(1) XOR frame(0)P(0)=column(0) COR line(1) XOR frame(0)

This set of equations generates a 4×4 spatial 4 bit spatial pattern asin FIG. 2e during the even frames and switches to a modulated version ofthe pattern as shown in FIG. 3 during the odd frames.

The pattern is temporally modulated in order to reduce the spatialvisibility of the pattern but the disadvantage is potentially someslightly visible flickering when using such a temporally varying patternas an offset to the video data prior to truncation. Therefore temporalmodulation of the pattern is not always desirable.

The effect of such a commonly used offset pattern dithering on the videosignal can be explained using the 2×2 pattern that was shown in FIG. 1c. Suppose video data is rendered at 10-bits per pixel per colour and isto be transmitted via an 8-bit DVI link. Imagine a dark sky beforesunrise. The sky contains a subtle and continuous change in luminosity.In this case the rendered steps are a quarter of the quantization levelto be applied to the signal prior to transmission. Imagine an area whereone colour channel, say blue, fades from 7 to 8. The following valuesare rendered by the image generator: 7, 7.25, 7.5, 7.75 and 8. Whentruncating these values without dithering to integer values, the resultbecomes 7, 7, 7, 7 and 8. The subtle colour changes are clearly lostbecause of the truncation needed to quantize the signal to 8-bits percolour per pixel. When the repetitive 2×2 pattern in FIG. 1c is added tothe video before the truncation operation, the result is different. Eachof the original video levels causes the output of the dither circuitryto vary spatially, as shown in FIGS. 4b-4d . When video=7 the outputshows no modulation or pattern, as shown in FIG. 4a . When video=7.25the constant input value in a certain area will appear as a modulatedvalue on the output of the dither circuitry as illustrated in FIG. 4b .Each 2×2 pixel area in the array of truncated pixel values has thecorrect average value. When video=7.5 the constant input value isdisplayed as a checker pattern as illustrated in FIG. 4c . Notice againthe correct average value of the truncated addition in the right column.This value is transmitted to the display device. When video=7.75 theresult is seen in FIG. 4 d.

Although the known dither patterns have the desired result on theaverage signal values of the video signal, the repetitive nature of thepattern has a discrete spectrum with high energy peaks, especially inthe high frequencies. In some cases additional video processing isrequired after dithering has already been applied to the video signal.Some processing functions, such as digital filters or interpolations,can interfere with the dithering patterns and artifacts such as falsecontouring and/or banding can become clearly visible. The ditheringprocess is often configured by a device manufacturer and is part of theinternal processing of a device, such as an image generator, display orprojector. Accordingly, it is not possible for a user to control theinterference effects that occur in the image processing chain.

The known dither patterns used in image processing applications,especially where dithering which is applied immediately before bit depthreduction, are small patterns (e.g. 2×2 pixels or 4×4 pixels) as it hasbeen considered that the pattern should have the effect of achieving anaverage value across as small an area as possible. A side effect of asmall repetitive dither pattern is that the resulting spectrum is adiscrete spectrum with high energy peaks, especially in the highfrequencies.

SUMMARY OF THE INVENTION

A first aspect of the present invention seeks to provide good,preferably improve processing of an image signal, where ditheringpattern is applied in a processing chain.

A second aspect of the present invention seeks to achieve a good,preferably an improved dithering process.

A first aspect of the invention provides a method of processing adigital image signal in a processing chain between an image generatorand an image display, wherein a dither pattern is applied to the imagesignal at a point in the processing chain, the dithering signal having afirst spectrum, the method comprising:

applying a noise pattern to the image signal in the processing chain,the noise pattern having a second spectrum which is configured such thatthe combination of the first spectrum and second spectrum results in amore continuous spectrum.

This aspect of the invention provides a way of lessening the potentiallydisturbing spectral peaks in the dither pattern used in the processingchain by adding a well designed and proper amount of entropy to thevideo signal. Another way of considering the effect of this aspect ofthe invention is that it “fills up” the gaps in the spectrum in betweenthe existing spectral peaks of the dither pattern. The resultingspectrum is a coloured (advantageously blue) noise spectrum. Thecontinuity of the energy distribution helps to ensure interference-freeimage processing on such a treated dithered signal.

This aspect of the invention helps to eliminate undesired artifacts byadding well controlled entropy to the dithered image. This removes theundesired properties of the dithered image that cause the interferencesand replaces the peaked spectrum by a more desirable continuousspectrum. In most cases a pattern offset dithering technique causesdiscrete spectral peaks where the energy increases with the frequency.Such an energy peaks can be considered as a fraction of a continuouscoloured noise spectrum. This can be cyan, blue or purple noise or anyother “cold” colour. Often the peak energy is proportional to thefrequency. In that case the discrete peaks can be considered as samplesfrom a blue noise spectrum. A digital signal can be generated and addedto the dithered image such that a blue noise spectrum is obtained. Acoloured noise spectrum is generated which is complementary to thepeaked dithering spectrum such that when the two signals are mixed acontinuous coloured noise spectrum is obtained. This eliminatespotential interferences with other digital signal processing.

It will be appreciated that the noise pattern will not have a spectrumwhich is perfectly continuous but will, itself, comprise a set of peaks.However, the peaks of the noise pattern advantageously are more closelyspaced and more numerous compared to the spectrum of the dither pattern.This helps to complete the gaps in the spectrum of the dither pattern.The spectrum of the noise pattern can be made more continuous as thesize of the noise pattern is increased. Advantageously, the ditherpattern is an A×B array of offset values which is tiled across the imageand the noise pattern is an N×M array of offset values which is tiledacross the image, where N×M>A×B.

The image signal can represent a digital still image or can comprise aframe of a video signal. In the case of a video signal, the noisepattern can be varied temporally. Temporal variation can be achieved bymodulating a noise pattern over a plurality of frames of the videosignal or by storing a set of different noise patterns which aresequentially applied to frames of the video signal.

Advantageously, a different noise pattern is used for each colourcomponent of the image signal. The colour components can comprise, forexample: YUV; Y, R-Y, B-Y; or RGB. The noise patterns can be made“different” by modulation of a single noise pattern, or by using adifferent stored pattern per colour component.

A particularly advantageous way of applying the noise pattern to theimage signal is by storing the noise pattern as an array in a look-uptable and accessing elements of the array as a function of the line androw number of the image signal. The noise pattern can be applied beforethe dither pattern in the processing chain (pre-processing scenario) orafter the dither pattern in the processing chain (post-processingscenario).

A second aspect of the invention provides a method of processing adigital image signal comprising applying a noise pattern to the imagesignal, the noise pattern comprising an array of offset values, whereinthe noise pattern has the following properties:

the values are linearly distributed across a range of values, with eachvalue in the range occurring a substantially equal number of timeswithin the array;

similar values at extreme ends of the range of values are dispersedwithin the array;

a substantially Poisson-disk two-dimensional spectral energydistribution; and wherein the noise pattern also has at least one of thefollowing further properties:

values are positioned in the array based on distance to similar valuesin neighbouring repetitions of the array;

a sum of any column of the array is substantially equal to a sum of anyother column of the array and a sum of any line of the array issubstantially equal to a sum of any other line of the array.

Using a noise pattern with these properties as a dither pattern in imageprocessing has been found to result in a processed image signal with amore continuous spectrum. This helps to reduce undesirable artifactswhen the processed signal is subsequently processed by other processingstages in an image processing chain. Advantageously, the noise patternis applied before a colour bit depth reduction operation.

It will be appreciated that a small deviation from strict compliancewith one or more of the listed properties will incur a small penalty inthe quality of the resulting spectrum. As an example, consider theproperty of a linearly distributed range of values, with each value inthe range occurring a substantially equal number of times. It will beapparent that it would be possible to use the same value twice (therebynot strictly meeting the requirement for a linearly distributed range ofvalues), and to not use another value at all. As another example,consider the “magic square” property where the sum of each line orcolumn is the same as any other line or column. A small deviation fromthe ideal sum in one or more of the rows or columns is not likely tohave a visible effect on the image signal. Accordingly, small deviationsare intended to fall within the scope of protection of the claims, andespecially deviations which do not make a visible effect on the imagesignal.

Further aspects of the invention provide methods for generating noisepatterns for use in the above methods.

The functionality described here can be implemented in software,hardware or a combination of these. The invention can be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. Accordingly, a further aspect of theinvention provides a digital image or video processing device comprisinga module for applying a noise pattern to a digital image or videosignal. A still further aspect of the invention provides software forperforming any of the described methods. The software may be tangiblyembodied on an electronic memory device, hard disk, optical disk orother machine-readable storage medium. The software may be delivered asa computer program product on a machine-readable carrier or it may bedownloaded to a processing device via a network connection.

Throughout the following description of the invention, the term “imagesignal” applies to a signal representing a still image or a signalrepresenting video.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the accompanying drawings in which:

FIGS. 1a-1c show a known 2×2 offset dither pattern that can be appliedto an image signal;

FIGS. 2a-2e show a known 4×4 offset dither pattern that can be appliedto an image signal;

FIG. 3 shows a modulated form of the offset dither pattern of FIG. 2ewhich can be used for temporal modulation of an image signal;

FIGS. 4a-4d show the effect of a 2×2 offset dither pattern on videodata;

FIG. 5 shows an 8×8 array of values for producing white noise which hasbeen generated without constraints;

FIG. 6 shows a 16×16 array of values for producing white noise which hasbeen generated with a linear distribution constraint;

FIG. 7 shows a result of using the noise pattern of FIG. 6 on videodata, and undesired graininess;

FIG. 8 shows a 16×16 array of values for producing blue noise;

FIG. 9 shows an improved 16×16 array of values for producing blue noisewhich has been generated with a constraint which places high and lowvalues before middle values;

FIG. 10 schematically shows apparatus for applying the noise pattern toan image signal using a look-up table and line and column counters;

FIG. 11 shows tiling of the noise pattern across an image;

FIG. 12 shows tiling of a noise pattern and another noise (dither)pattern across an image;

FIG. 13 shows a desired two-dimensional spectrum of the noise patternapproximating a Poisson disk;

FIG. 14 shows a 16×16 array of values resulting from an additivesynthesis process;

FIG. 15 shows a further 16×16 array of values resulting from an additivesynthesis process with additional constraints compared to FIG. 14;

FIG. 16 shows a two-dimensional spectrum resulting from an array whichhas been generated using an additive synthesis process;

FIG. 17 shows a 1D spectrum to illustrate the process of configuring thespectrum of the noise pattern to complement the spectrum of anotherdither pattern;

FIG. 18 schematically shows post-processing an image signal, to whichdither has already been applied, with the new noise pattern;

FIG. 19 schematically shows pre-processing an image signal with the newnoise pattern before another dither pattern is applied to the imagesignal;

FIG. 20 shows a 64×64 array of values generated using an additivesynthesis process with additional constraints;

FIG. 21 shows a two-dimensional spectrum corresponding to the array ofFIG. 20; and,

FIG. 22 shows an example image processing chain in which the inventioncan be used.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps. Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

White Noise Generation

A computer program can calculate any coloured noise signal. The easiesttype of noise to create is white noise. Two dimensional white noise witha certain bit depth can be generated by producing a sequence of standardfloating point random numbers then truncating the results according tothe desired bit depth and storing them in a two dimensional array. Thisproduces white noise without any constraints.

In case 16-bit white noise is desired a 256×256 array can be used tostore the integer numbers in a range from 0 to 65535. To generate 8-bitwhite noise a 16×16 array is suitable to store the integer numbers in arange from 0 to 255. When no constraints are applied it is likely thatsome numbers will appear twice or more in the array and consequently itis also likely that some numbers will not appear at all and this willcause an undesired non-linear behaviour of the entire truncationprocess. This is illustrated in FIG. 5 for a 6-bit array of 8×8 pixels.As can be seen the value zero occurs 3 times, value 1 only once andvalue 3 for instance is missing. This noise pattern could be used toreduce video data from 16-bit to 10-bit. However, equally spread 16-bitincrements of the dithering input data will cause different steps at the10-bit output. Some steps will be magnified by a factor of three or evenmore as some values appear at least three times in the array in FIG. 5,while other inputs steps will not cause any changes to the output of thedither circuit. This is a much undesired non-linear behaviour that caneasily be solved by adding a constraint when the numbers are generated.Each time a random value is generated one can verify whether or not thevalue was generated before. In that case a new random number isgenerated until the result is a new unique value. In this example thefinal result will be an 8×8 array with all unique values in an integerrange from 0 to 63.

Another approach is by inserting all numbers from 0 to 63 sequentiallyat random positions in the 8×8 array. When a position is occupied by aprevious number a new random position is generated until a previouslyunused position in the array is found. Then the value is stored at thisposition. In this way the final result is again an 8×8 array with allunique values in the integer range from 0 to 63.

The BASIC program in Annex A illustrates a possible implementation ofthe above described process for an 8-bit white noise pattern. The resultis a two dimensional array (pattern) that contains the full range of8-bit integer numbers. The spectrum generated with this process iswhite.

Although this dither method has a lot of interesting properties such asa linear behaviour and a continuous spectrum which avoids potentialinterferences it causes the images to appear “grainy”. This can beindicated using FIG. 6 which shows a result of the program explainedabove.

Suppose a video value of 251 is added to the noise pattern shown in FIG.6. When the result is divided by 256 and then truncated most of thepixels will have a value of 1, but some will be truncated to 0. A valueof 251/256^(th) is converted by the dither process to 251/256 of thepixels having a value of 1 and 5/256 of the pixels having a value of 0.This way the correct average value is obtained in each 16×16 pixelarray. The result is shown in FIG. 8.

As can be seen in FIG. 7, a video level of 251 introduces small clustersof black dots. This explains the grainy appearance of images ditheredwith white noise. These clusters represent low frequency energy presentin the spectrum.

Blue Noise Generation

To avoid the image graininess typical for white noise based ditheringmore constraints can be added to the above mentioned method. These extraconstraints will colour the noise pattern. While scanning all values,constraining the minimum distance between the location of the previouslyscanned values and the current value to be placed in the array is asimple approach to create a bluish noise spectrum.

The BASIC program in Annex B illustrates a possible implementation ofthe above described process for a first order approach of an 8-bit bluenoise pattern. The program scans all values in a range from 0 to 255 andplaces them in the blue noise pattern array. As the array gets more andmore filled up the minimum distance constraint needs to be adapted. Thisis done by tracking the maximum possible distance from any previouslyplaced value per array location. For each element the distance to thenearest used location (DTNUL parameter in the program) is tracked. Themaximum of the DTNUL values in the entire array is the largest distanceto any used location (LDTAUL parameter in the program). This LDTAULparameter is refreshed each time a value has been placed in the array.

Each time a random location is generated the DTNUL value is compared tothe LDTAUL value for that location. In order for the new random locationto be valid for the current noise value the DTNUL value must be above aparameterized fraction of the LDTAUL value. This threshold parameter hasa useful range between 0 and 1. The threshold value has an influence onthe noise colour. Very high values close to 1 will bound the randomnessof the generated array and this will produce patterns similar to theexample in FIG. 3 and thus an undesirable high frequency peakedspectrum. Very low values close to 0 will not constrain the randomnessof the generated array and thus will produce white noise patternssimilar to the example in FIG. 7 which causes an undesirable graininessof the image. As the threshold value has a major impact on the obtainednoise spectrum its value has to be carefully chosen or calculated. Somethreshold values produce interesting properties. Some examples are thegolden ratio number (0.618) and the square root of a half (0.707) andmore advanced calculations have been found to yield numbers in a rangetypically between 0.6 and 0.9.

For each array element (x,y) the distance to the current location(xc,yc) is calculated by the program after each new valid placement of anoise value in the current location xc,yc. The distance is calculatedas:distance=Sqrt((x−xc)²+(y−yc)²).

However, as the pattern repetitivity has to be taken into account the socalled pan distance needs to be calculated. The pan distance betweenlocation A and B in the array is the smallest distance that can be foundbetween all As and Bs when the pattern is repeated in 2 dimensions. Forexample, in a 64×64 array the distance between A in the left toplocation and B in the right bottom location=squareroot (63²+63²).However, the pan distance is only squareroot (1+1) as the A location ofthe repeated pattern (found in the next column and row of patterns) isvery close to the B location in the current pattern. The pan distancebetween A and B effectively is the minimum distance between all As andall Bs in the entire image. The pan distance ensures that occupiedlocations near the edges of the array are contributing to theconstraints of the noise values placement in the neighboring arrays.These neighboring arrays are an exact duplicate of the current arraybeing calculated because of the pattern repetition.

The pattern above shows well distributed low values, but the highervalues indicate some clusters. Values 253 and 255 are located near eachother causing undesired graininess again. The algorithm spreads lowvalues very well but as the array gets filled up there is no room leftfor the higher values to be spread out as desired. This can be solved byalternately placing a low noise value and a high noise value in thearray. Such a sequence can be 0, 255, 1, 254, 2 . . . 127 and 128. Thespectral energy distribution is mainly determined by these first extremevalues.

The distance to the nearest used location must now be tracked for thelow and high values separately. This adds an extra dimension to theDTNUL array which now becomes three-dimensional. The largest distance toany used location must also be tracked for low and high noise valuesseparately. Therefore the LDTAUL also becomes an array. A possibleimplementation of such an algorithm is illustrated in the BASIC programbelow.

The BASIC program in Annex C generates a blue noise pattern with noclusters in low values or in the high values. Naturally, clusters withnumbers near the centre value (127.5 in case of an 8-bit noise pattern)can occur but as these represent almost no modulation energy theseclusters hardly contribute to the undesired low frequencies in thespectrum. A typical output of the described algorithm is shown in FIG.12 for a 16×16 pixel pattern. The pattern in FIG. 9 can be mixed withimage or video data almost without introducing interferences. The largerthe pattern is chosen the more continuous the spectrum can be made andthus the better interferences can be avoided. Larger patterns willrequire more memory for storage and thus hardware implementationsrequire more resources.

Preferred Embodiment for Coloured Noise Dithering

FIG. 10 shows an embodiment of an image/video processing chain in whicha noise pattern is applied to an image/video signal. A repetitivepattern is desirable for a real time processing hardware implementationas it reduces the required hardware resources. The noise pattern isstored in a look-up table. In hardware implementations the memorydensity required to store the pattern depends on the size of thepattern. In the case of a 64×64 pattern each noise bit requires 4 Kbitof embedded RAM. For an N-bit noise pattern, the total memoryrequirement is 4K×N=N Kbit of memory. The look-up table is accessed byforming a read address based on the line and column of the image. As theline counter and column counter are incremented to scan the pixels ofthe image, a read address is formed and an element of the look-up tablecorresponding to that read address is read out. Data from the look-uptable is added to data bits of the image signal. The scheme shown inFIG. 10, where a read address is formed using N least significant bitsof the line counter and M bits of the column counter is an example. Manyother schemes are possible.

FIG. 11 illustrates the process of tiling the noise pattern 10 acrossthe image 12. In this example the image is 16 pixels×16 pixels and thenoise pattern is an 8×8 array of values. The same noise pattern 10 istiled across the image in locations 1, 2, 3, 4. At each location, anelement of the noise pattern is added to a pixel value. This is repeatedfor every noise pattern element/pixel pair. FIG. 10 shows that thetiling can be implemented by a look-up table based on column and rowcounter values.

In the examples so far the pattern has been illustrated as a square.Although an N×N square array containing N² unique values is particularlyadvantageous, a rectangular N×M array containing N×M different valuescan also be used. For example, a 128×64 pattern would provide goodresults. As explained earlier, it is desirable to equally distribute thenoise values to obtain a linear quantization. All possible values withinthe desired range should appear. Therefore the maximum noise data widththat can be read from the M×N noise pattern look-up table is M+N bits.In the case of a 64×64 pattern the maximum noise pattern precision is 12bit.

When the noise pattern is added to the video data with a precision of Vbits, it is possible to obtain a result larger than the highest numberrepresentable by V bits. In this case the adder (indicated by the + signin FIG. 10) generates a so-called overflow bit. The clip protectionblock in FIG. 10 performs a logic OR function to maintain a white outputwhen the well controlled noise signal as proposed by the currentinvention is added to a white input. Without this clip protection addingnoise to a white input would introduce highly undesirable blackspeckles.

FIG. 12 shows an advantageous relationship between the size of the noisepattern 10 and a dither pattern 14 which may also be tiled across theimage at some point in the image processing chain. It is desirable thatthe noise pattern 10 has a larger number of elements compared with thenoise pattern as the larger number of elements can help to provide amore continuous spectrum.

The algorithm described in Annex C generates a continuous blue noisespectrum. The two-dimensional spectrum approximates a so-calledPoisson-disk distribution as illustrated in FIG. 13. Due to theconstraints applied to the distances between successive noise valuesduring the placement algorithm any graininess in the image is avoided.In two dimensional spectral terms this means that there is no energypresent in the frequency coordinates within a certain radius near thecentre. The centre represents the point without any oscillations in bothdirections.

The Poisson-disk energy distribution indicates that all energies aredistributed equally over high frequencies in any dimension. In each ofthe two dimensions the amount of energy present in the noise signalincreases with the frequency as more frequencies in the other dimensiondeliver a contribution to the total energy.

The Poisson-disk distribution is known as the best sample pattern toavoid any interference with image data. This is not only based onmathematical properties as discussed above but is also based on theevidence that can be found in our own eyes. The Poisson-diskdistribution is found among the sparse retinal cells outside the fovealregion in the human eye. Although the human brain intensively processesthe captured image causing some remarkable artifacts of totallydifferent types the pattern of these photo receptors never causes anynoticeable interference or aliasing with details or repetitive patternsimages you can see.

Controlling the Coloured Noise Spectrum

Another approach to generate a Poisson-disk distributed noise pattern isby additively synthesizing the spectrum. Each two dimensional frequencycan be calculated with its own independent two-dimensional phase andamplitude. This is illustrated by the BASIC program in Annex D for a16×16 pattern. In this example all discrete spatial frequencies that canoccur in a 16×16 pattern are calculated for each element in the 16×16array as floating point numbers. The two dimensional spatial frequenciesabove a specified Poisson radius parameter multiplied by the highestpossible frequency are calculated with random two-dimensional phase (px,py) and constant unity amplitude (ax,ay).

For a 16×16 pixel pattern the range of the oscillation period is from 2to 16 pixels. The longest period available is 8 times the shortestperiod which is considered as the spectral unity in the program. Theprogram synthesizes 8×8 frequencies. It does not compute any frequencycomponents equal to zero. This is to avoid certain columns or lines tobe brighter in average than others. In order two have equal averagebrightness of all columns and lines the average noise value added percolumn or per line should be equal. This is similar to constructing amagic square. If the pattern does not contain spectral frequencieslocated on the x and y axis in the two-dimensional Fourier transformthis condition is best fulfilled. In general, approaching magic squareproperties as much as possible is desirable as these propertiescontribute well to the noise signal entropy. These magic properties willbe explained in more detail further on.

Special attention is required for the highest available frequency ineach dimension. As a pixel on-pixel off pattern is not attenuated by aresampling process a correction is needed to accommodate for thedifferences with other frequencies. The best correction value to use isthe golden ratio as illustrated in the program and can be determinedusing statistical equations.

The BASIC program in Annex D outputs a 16×16 pattern with unequallyspread floating point numbers. In order to convert the obtained valuesto an integer range from 0 to 255 an additional function must beperformed to obtain the desired range of integer numbers. Obviously suchan operation is equivalent to a non-linear quantization and will alterthe obtained spectrum. As a result the integer pattern will have a lessperfect Poisson-disk distribution. It is however a good starting pointfor further computations as will be explained later.

Annex E illustrates how a series of unequally distributed floating pointnumbers can be converted to an equally spread integer range of numbers.Annex E scans the desired 8-bit integers from 0 to 255 and locates thefloating point values from minimum to maximum value. Due to the largenegative offset value the floating point values were initialized earlyin the program in Annex D it is possible to replace the minimum value bythe currently scanned integer value while the unprocessed valuesmaintain negative numbers.

The floating points generated by the program in Annex D are all largenegative numbers while the program in Annex E outputs positive integernumbers. This way only one array is required to perform the numberformat conversion as can be seen in Annex E. Once the minimum negativefloating point value is replaced by 0 the second largest negative numberbecomes the minimum value and will be replaced by 1. This processcontinues until all 256 values are replaced by integers.

Despite the well approximated Poisson-disk distribution of the spectrumthe algorithm illustrated in Annexes D and E does not evenly diffuse thepeak values properly as is the case with the algorithm illustrated inAnnex C.

A typical example of the result generated by the program illustrated inAnnexes D and E is shown in FIG. 14. This shows that the peak values arenot evenly diffused among the noise pattern. Although this does notnecessarily introduce interference artifacts it is less desirable as thenearly clustering lowest peak values indicated in the Figure potentiallycause graininess in the image.

It is possible to combine the advantages of both methods that wereillustrated in FIGS. 9 and 14. Although the algorithm used to constructthe noise pattern in FIG. 9 has a threshold parameter to control thenoise colour it does not allow to precisely control the spectrum. Itdoes however produce a noise pattern that has many desirable propertiesas described earlier. On the other hand the algorithm used to constructthe noise pattern in FIG. 14 has a much more controllable spectrum as itperforms additive synthesis where each of the two-dimensional frequencycomponents which compose the coloured noise spectrum has controllableamplitude. Although these amplitudes are slightly altered by thenon-linear conversion process from the original floating point values tothe desired uniformly spread integer range the final energy distributionin the spectrum very well approximates the targeted spectrum.

The algorithm illustrated for a 16×16 pattern in Annex C uses athreshold parameter to constraint the random placement of successivenoise values. After the location for value 0 is determined a minimumdistance constraint is applied for the placement of value 1. When a newrandom position is calculated the minimum distance constraint is testedand based on this comparison the new location is accepted or rejected.Similarly, an algorithm can look-up the location of value 0 in asynthesized coloured noise pattern as illustrated in FIG. 14 for a 16×16pattern. Then the same minimum distance constraint as mentioned abovecan be used to determine the best possible position for value 1.

The minimum distance constraint is tested for all free locations in thearray and based on this comparison the new location is marked as a validor invalid location for the value 1. Then the minimum value of alllocations marked as valid is located in the array as illustrated in FIG.14. This process is repeated for all values up to 127 in the example ofa 16×16 pattern.

The same process is repeated for the upper values starting by locatingthe location of value 255 and determining the best position in the arrayfor value 254 based on the same minimum distance constraint determinedby the threshold parameter. In this 16×16 pattern example the last valuethat will be placed into position is 128. A typical result of the abovedescribed algorithm can be found in FIG. 15. Although the spectrum isvery similar to the spectrum of the pattern obtained by non-linearlyquantized additive synthesis as illustrated in FIG. 14 the peak valuesare much better diffused among the pattern.

The BASIC program in Annex F illustrates the above described techniqueto generate a Poisson-disk distributed spectrum with uniformly diffusedpattern peak values to avoid graininess in the image. The programperforms additional computations to the noise pattern generated by theprogram in Annex E. The program first offsets the lower 128 and higher128 values separately to simplify the additional computations needed tofind the optimal location for each value in the array as explained. Thenthe program continues by scanning the first half of the desired integervalue range by taking into account the minimum distance thresholdparameter based on the additively synthesized coloured noise spectrum.Before scanning the upper half of the integer range the programreinitializes the DTNUL and LDTAUL values as the spacing between highvalues is independent from the distribution of low values.

The result of the program in Annex F is an equally distributed noisepattern with a spectrum close to the optimal Poisson-disk distribution.A typical example of the obtained spectrum by this algorithm is showFIG. 16. As the spectrum is symmetrical around the two frequency axisthe Figure only shows its positive quadrant. In this example a 64×64coloured noise pattern was calculated with 12 bit integer values usingthe algorithms described in Figures D, E and F.

The spectrum shows the entire range of frequencies a repetitive 64×64pattern can contain with periods from 2 pixels to 64 pixels. It alsoincludes the frequencies on the axis equal to zero. Therefore 33×33frequencies are calculated for a 64×64 coloured noise pattern. Asclearly visible by the quarter circle the spectrum accuratelyapproximates the Poisson-disk distribution.

A possible practical implementation is already shown in FIG. 10.Consider an image generator renders an image with 20 bit per colouraccuracy. After adding the 64×64 coloured noise pattern with a spectrumas displayed in FIG. 20 the video data can be truncated to 8-bit percolour. This 8-bit signal represents the original 20 bit signalpractically without introducing any interference or other undesiredartifacts. The size of the look-up table can be easily calculated. 64×64pixels require 4096 or 4K addresses. The data bus is 12-bits wide. Thetotal memory resources required are only 4K×12bit=48 Kbit of memory. Intoday's FPGA devices with several megabits of embedded memory thisdither circuit requires only a fraction of the available deviceresources. It will be appreciated that the processing described in thisinvention can be performed by Digital Signal Processing (DSP) chips orother integrated circuits, a hardware implementation using discretedevices or a processor executing software.

So far only one colour has been considered. There are a fewpossibilities to process multiple colour spaces such as: YUV; Y, R-Y,B-Y; or RGB. One simple approach is to invert the noise pattern valuesbefore adding them to the green video channel. When the red and bluechannels receive the non-inverted noise pattern the luminancefluctuations can be minimized. Another possibility is to generate aseparate noise pattern per colour. The optimal combination for noisepatterns per colour component depends on the colour space used.

Temporal modulation can be applied to the spatial coloured noise patternto minimize its visibility. The pattern can be extended to athree-dimensional look-up table, although it will be appreciated thatthis has a penalty of requiring more resources to implement the circuit.A more efficient approach is to modulate some aspects of the patternbased on the frame counter. Some operations can be applied to thecoloured noise pattern while entirely preserving its interestingspectral properties. These operations include shifting or translatingthe pattern in both spatial dimensions, rotating the pattern, mirroringthe pattern in both spatial dimensions and inverting the values.

Shifting the pattern dynamically in horizontal dimension can be achievedby applying a XOR function with any of the least significant framecounter bits to the most significant column counter bit applied to thelook-up table in FIGS. 10, 18 and 19. Similarly shifting the patterndynamically in vertical dimension can be done by applying a XOR functionwith any of the least significant frame counter bits to the mostsignificant line counter bit applied to this look-up table. Bydynamically inverting the most significant column counter bit the noisepattern is shifted horizontally by half its width. This is equivalent toadding half the width to the column counter. Dynamically inverting themost significant line counter bit shifts the noise pattern vertically byhalf its height. This is equivalent to adding half the height to theline counter. The XOR implementation is not only more efficient comparedto a full adder but also adds a desirable amount of shifting by half thewidth and/or height. Rotating the pattern dynamically can be based onswapping the column and line counter as function of a certain framecounter bit.

Mirroring the pattern dynamically in horizontal dimension is performedby a XOR function with any of the least significant frame counter bitson all column counter bits applied to the look-up table simultaneously.Similarly mirroring the pattern dynamically in vertical dimension isperformed by a XOR function with any of the least significant framecounter bits on all line counter bits applied to the look-up tablesimultaneously.

Although the temporal modulation possibilities are not restricted to theexamples mentioned above the mentioned examples are amongst the mostefficient and most qualitative methods for temporal modulation of anoise offset pattern.

Matching the Coloured Noise Spectrum to the Offset Dither Pattern

The noise patterns described so far can be applied to an image or videosignal as an optimised form of dither, to provide better resultscompared with conventional small patterns. It is also possible to usethe noise patterns described previously even where another device in theprocessing path performs a dithering operation. By matching the spectrumof the noise pattern to complement the spectrum of the other ditherpatterns, such as those described in FIGS. 1, 2 and 3, it is possible toachieve a more continuous overall spectrum which is better suited toother image processing operations that may occur in the processingchain. An additional computation stage is required to reduce the localenergy in the synthesized spectrum using the algorithm as illustrated inAnnex F. The program in Annex G applies these extra constraints to thesynthesized spectrum.

Considering again the dithering pattern shown in FIG. 2e , the spectrumcorresponding to this pattern has 4 energy peaks. Their two-dimensionalspectral positions and magnitudes can be calculated using the 2D Fouriertransform:

Peak(1) : Energy=1, Fx=1, Fy=1

Peak(2) : Energy=0.5, Fx=1, Fy=0

Peak(3) : Energy=0.125, Fx=0.5, Fy=0.5

Peak(4) : Energy=0.088, Fx=1, Fy=0.5

Where Fx and Fy are the spatial frequencies relative to the Nyquistfrequency.

These frequencies are excluded, or suppressed, completely by the programin

Annex G to obtain a smooth continuous mixed spectrum. The optimisedcoloured (blue) noise pattern generated with the combination of thealgorithms described in Annexes F and G avoids potential interferencecaused by an earlier or later applied dither technique in the processingchain. FIG. 17 illustrates the effect of the matching process using a 1Dfrequency plot. Although the noise pattern is configured for 2Dfrequency, the plot of FIG. 17 helps to more clearly illustrate theprinciple of the matching process. The existing dither process has apeaked spectrum 22. Spectrum 22 can have more peaks than are shown here,with locations defined in the 2D frequency domain. The spectrum 20 ofthe noise pattern is configured with minima at frequencies correspondingto frequencies at which the peaks in spectrum 22 occur. FIG. 17 shows ablue noise spectrum which increases with frequency.

The noise pattern can be applied to an image signal before anotherdithering process (the pre-processing scenario) or it can be applied tothe image signal after another dithering process has been applied to thesignal (the post-processing scenario).

The post-processing scenario is illustrated in FIG. 18. Conventionaldither techniques mix the image or video data with a noise patterncausing the video output to alternate between 2 successive values. Thismeans that the final noise pattern amplitude that appears on the outputvideo data must be considered as equal to the quantizing error. In orderto fill up the energy gaps between the spectral peaks caused by such adither technique it is therefore sufficient to add a matched colourednoise pattern with 1 bit amplitude per colour. The array of valuesstored in the look-up table of FIG. 18 is exactly as describedpreviously, except that there is only a need to use the most significantbit of each entry in the array. Indeed, it is possible to reduce thecontents of the LUT and store just the MSB bit of each value.

The video out signal in FIG. 18 can be subjected to various kinds ofdigital image processing, including filtering and interpolations withoutthe risk of introducing artifacts caused by interference with the ditherpattern present in the video input signal. The pre-processing scenariois illustrated in FIG. 19. An image or video stream is pre-processed tohelp eliminate interference artifacts introduced by the pattern baseddithering further on in the processing path. A well-controlled colourednoise signal, as described earlier, is added to the video data.Advantageously, the noise amplitude and thus the amount of noise bitsadded to the video data matches the amount of bits reduced by thepattern based dithering. In many cases the pattern based dithering isperformed by a display driver chip. A practical example clarifies thismethod.

Suppose a display device can handle 10-bit video data per colour.Imagine a 14 bit video processing block sends a 14 bit data stream percolour to a display driver chip performing built in dithering to 10-bit.In this case the current invention adds a 4 bit coloured noise signal tothe video data stream. The M×N coloured noise pattern stored in alook-up table is complementary to the spectrum introduced by the 4 bitpattern based dithering inside the display driver circuitry. A possiblealgorithm to generate the look-up table contents has already beendescribed in Annexes F and G.

Due to the increased entropy of the image or video data the entire imageprocessing system (including image source, optional video processing,display driver circuitry and display device) is capable of displayingmore image information more accurately without artifacts.

Some techniques for matching the spectrum of the noise pattern to thespectrum of the dither pattern will now be described.

(1) In many cases, details of the type of spatial dither pattern areprovided in documentation provided by the manufacturer of a processingdevice. By calculating the 2D Fourier transform of the specified patternthe spectral energy peaks can be detected. Usually only a fewfrequencies have relevant values. The table shown above for the patternof FIG. 2e is a typical set of high energy peaks. The spectrum of thenoise pattern is modified based on this set of energy peaks.

(2) In a case where the dither pattern is not specified it can bemeasured in several ways. A uniform grey level signal is applied to adevice in the processing chain, prior to the dither circuitry. Themodulated output is measured. By increasing the uniform input grey levelby 1, only 1 pixel in the pattern will light up one step more at theoutput, indicating the value that was offset by the dither circuit forthat pixel.

(3) Another way is to macroscopically look at the screen while applyingthe suspected dither pattern prior to dithering. Consider one wants todetect the most significant bit of the dither pattern. Assume thesuspected equation is line(0) XOR column(0). When this image isgenerated a checker pattern occurs. Assume the generated image hasvalues alternating between 0 and 0.5. In case the dithering pattern isin phase with the generated input pattern, the image lights up. In casethe dithering pattern is in counter phase with the generated inputpattern, no effect is visible. This proves the dither pattern equals thesuspected pattern. Such a system could be used in an automaticcalibration setup when using a light sensor that communicates with a PC.A PC can generate a set of test images and while communicating with alight measuring tool. It can detect the pattern that causes mostvariation of light output when the pattern phase is inverted. Thispattern best matches the dither pattern.

Optimising the Look-Up Table Contents

The (optionally) temporally modulated “magic” properties containingcoloured noise pattern can be considered as the inverse entropy of thepattern based dither process present in many practical systems usedtoday as indicated in FIG. 19. The desired spectrum is converted to anM×N impulse response kernel and this M×N noise pattern is encrypted intoa look-up table contents. Applying the least significant bits of columnand line counter to the look-up table read address bus will cause theread data to behave as the desired noise pattern.

In order to optimize the entropy of the coloured noise pattern it isdesirable to optimize its magic properties. As the base for the additivesynthesis as described in Annex G does not produce any DC signals andincludes multiples of the pattern frequency uniquely the floating pointoutput consequently has some desirable magic properties.

As no DC signal is present in the two spatial dimensions the sum of allnoise values in any column or any line equals zero. By applying thequantization process as described in Annex F these magic propertiesbecome disturbed. Although the sum of all elements in a specific line orcolumn remains nearly constant after the described quantization processto integer values these sums are not guaranteed to be constant becauseof the non-linear conversion from floating point values to integers.Because of the random initializations in some part of the program thecoloured noise synthesis algorithm as described in Annexes F and Gproduces a different result each time the code is executed. Thesedifferent results include slightly different spectral properties as wellas slightly different magic properties.

The algorithm can be executed millions of times by a PC and the bestresult can be picked from millions of generated coloured noise patterns.

The spectral quality can be calculated based on the aberrance comparedto the ideal Poisson-disk distribution using the least square errormethod for instance.

The magic properties can be quantified by adding the deviation from theideal sum for all columns and lines. Regularly the algorithm produces acoloured noise pattern with constant sum of columns and sum of lines.

Naturally additional computations could further constrain the algorithmto obtain specific magic properties more efficiently although theefficiency of such an algorithm is not really important as the finalresult is stored in a look-up table. Whether it takes a few seconds or afew days for a computer program to obtain a pattern with every desiredproperty does not influence the final real time performance or thequality improvement achieved by the current invention.

Therefore a more detailed discussion about computational efficiency toobtain a valid noise pattern must not be considered as part of thecurrent invention.

A typical example of a coloured noise pattern complementing a standarddither method obtained after hundreds of executions of the program inAnnex G followed by the code in Annex F is shown in FIG. 20 for a 12 bit64×64 pattern.

The locations of the upper half of the 12-bit integer numbers are markedin grey to provide a better overview of the value distribution. Byhighlighting the pattern's most significant bit the spectraldistribution becomes more apparent. FIG. 20 clearly indicates theabsence of any two-dimensional clusters in the pattern thus images mixedwith such a pattern do not appear grainy.

The spectrum of the pattern in FIG. 20 is shown in FIG. 21 and shows awell-approximated Poisson-disk distribution with an additional “magic”property as no energy is found on the axis in the chart indicating theabsence of any DC component on top of the average truncation value. Asthe pattern has 64×64 values all values in a range from 0 to 4095 areincluded. The average value for the pattern equals 4095 divided by 2.Thus the sum of each row and each column equals 64×2047.5=131040 as canbe verified in FIG. 20. This desirable property causes all video linesand columns to be equally quantized in average. Even without temporalmodulation of the pattern all lines and columns will be displayedequally bright in average.

Examples of noise pattern arrays have been described with a singleoccurrence of each value in a linear range, such as a 64×64 array of12-bit values in the range 0-4095 with each value occurring only once.It is also possible to create an array with multiple occurrences of eachvalue. For example, the same 64×64 array can store 9-bit values, witheach value in the range 0-511 ideally occurring 8 times. A simple way ofcreating these alternative patterns is by truncating the leastsignificant bits of the higher-bit values.

For completeness, FIG. 22 shows an example of an image processing chainin which the invention can be used. An image generator 30 is connectedto an image display 40, which in this case is a projector, by aninterface 35. Image generator 30 can comprise a media player with amedia content store 31 and a processor 32 to construct an image signal.A final processing stage 33 reduces the colour bit depth prior totransmission over interface 35. Processing stage 33 applies a repetitiveoffset dither pattern to the image signal before reducing the resolutionof the image signal. At projector 40, the digital image signal isreceived and a noise pattern is tiled across the image signal. The noisepattern has a spectrum which is complementary to that of the ditherpattern used by processing stage 33. Further processing is applied tothe image signal at stage 42. Due to the noise pattern that has beenapplied at stage 41, the signal input to stage 42 has a more continuousspectrum and the processing applied at stage 42 does not cause visibleartifacts. Finally, the digital image signal, in electrical form, isconverted to an optical signal for display 45.

The invention is not limited to the embodiments described herein, whichmay be modified or varied without departing from the scope of theinvention.

APPENDIXES

APPENDIX A *** BASIC Program to generate 16 × 16 white noise includinglinear quantization constraint *** For x = 0 To 15 ; scans all columnsof the array For y = 0 To 15 ; scans all lines of the array pattern(x,y) = −1 ; initializes all values to −1 to indicate “empty” location Nexty ; −1 will be used to detect empty locations Next x For Value = 0 To255 ; scans all values that must occur in the noise pattern  Do x =Int(Rnd * 16) ; generates a random column number y = Int(Rnd * 16) ;generates a random line number Loop Until pattern(x, y) < 0 ; loopsuntil an “empty” location is found pattern(x, y) = Value ; stores thecurrent value at the empty array location Next Point

APPENDIX B *** BASIC Program for first order approximation to generateblue noise *** For x = 0 To 15 ; scans all columns of the array For y =0 To 15 ; scans all lines of the array DTNUL(x, y) = 100 ; initializesall distances to the nearest used location pattern(x, y) = −1 ;initializes all values to −1 to indicate “empty” location Next y Next xLDTAUL = 100 ; initializes largest distance to any used location ForPoint = 0 To 255 ; scans all values that must occur in the noise patternDo ; loops until a valid location is found as explained below xc =Int(Rnd * 16) ; generates a random column number yc = Int(Rnd * 16) ;generates a random line number Loop until DTNUL (xc, yc) > LDTAUL *threshold pattern(xc, yc) = Point ; stores the current value at theempty array location LDTAUL = 0 ; reinitializes largest distance to anyused location For x = 0 To 15 ; scans all columns of the array For y = 0To 15 ; scans all lines of the array distance = Sqr((8 − Abs(8 − Abs(x −xc))) {circumflex over ( )} 2 + (8 − Abs(8 − Abs(y − yc))) {circumflexover ( )} 2) ; calculates the so called pan distance If DTNUL (x, y) >distance Then DTNUL (x, y) = distance ; tracks the distance to thenearest used location If LDTAUL < DTNUL (x, y) Then LDTAUL = DTNUL (x,y) ; tracks the largest distance to any used location Next y Next x NextPoint

APPENDIX C *** BASIC Program for blue noise generation *** For x = 0 To15 ; scans all columns of the array For y = 0 To 15 ; scans all lines ofthe array DTNUL(0,x, y) = 100 ; initializes all distances to the nearestlow values DTNUL(1,x, y) = 100 ; initializes all distances to thenearest high values pattern(x, y) = −1 ; initializes all values to −1 toindicate “empty” location Next y Next x LDTAUL(0) = 100 ; initializeslargest distance to any placed low value LDTAUL (1) = 100 ; initializeslargest distance to any placed high value For Point_pair = 0 To 127 ;scans all value pairs For Low_high = 0 To 1 ; this will alternatelyplace low and high value Point = Abs(255 * Low_high − Point_pair) ; whenlow_high is 1 values are scanned starting at 255 Do ; loops until thevalid location is “empty” Do ; loops until a valid location is found forlow or high value xc = Int(Rnd * 16) ; generates a random column numberyc = Int(Rnd * 16) ; generates a random line number Loop UntilDTNUL(low_high, xc, yc) > LDTAUL(low_high) * threshold Loop Untilpattern(xc, yc) < 0 pattern(xc, yc) = Point ; stores the current valueat the empty array location DTNUL(0, xc, yc) = 0 ; sets both DTNULvalues to zero as current location... DTNUL(1, xc, yc) = 0 ; ...isinvalid for both new low and high values LDTAUL(0) = 0 ; reinitializeslargest distance to any low value LDTAUL(1) = 0 ; reinitializes largestdistance to any high value For x = 0 To 15 ; scans all columns of thearray For y = 0 To 15 ; scans all lines of the array distance = Sqr((8 −Abs(8 − Abs(x − xc))) {circumflex over ( )} 2 + (8 − Abs(8 − Abs(y −yc))) {circumflex over ( )} 2) ; calculates the so called pan distance If DTNUL(low_high, x, y) > distance Then DTNUL(low_high, x, y) =distance ; tracks the distance to the nearest low or high location  Ifpattern(x, y) < 0 Then ; ensures that only empty locations areconsidered  If LDTAUL(0) < DTNUL(0, x, y) Then LDTAUL (0) = DTNUL (0, x,y) ; tracks the largest distance to any placed low value  If LDTAUL (1)< DTNUL (1, x, y) Then LDTAUL (1) = DTNUL (1, x, y) ; tracks the largestdistance to any placed high value End If Next y Next x Next Low_highNext Point_pair

APPENDIX D *** BASIC Program to synthesize a Poisson-disk distributedspectrum *** golden_ratio = (Sqr(5) − 1) / 2 ; defines the golden rationumber For x = 0 To 15 ; scans all columns in the 16x16 pattern For y =0 To 15 ; scans all lines in the 16x16 pattern pattern(x, y) = −10000 ;initializes the 16x16 noise pattern values the large negative offsetsimplifies further computations Next y Next x For Fx = 1 To 8 ; scansall horizontal frequencies Fx except 0 For Fy = 1 To 8 ; scans allhorizontal frequencies Fy except 0 If (Fx {circumflex over ( )} 2 + Fy{circumflex over ( )} 2) > (Poisson_Radius * 8) {circumflex over ( )} 2Then ; 2D-frequency must be above specified fraction of maximumfrequency px = Rnd(1) * 2 * Pi ; generates a random horizontal phase py= Rnd(1) * 2 * Pi ; generates a random vertical phase If Fx = 8 Then ax= golden_ratio Else ax = 1 ; defines horizontal attenuation ax If Fy = 8Then ay = golden_ratio Else ay = 1 ; defines vertical attenuation ax Forx = 0 To 15 ; scans all columns in the 16x16 pattern For y = 0 To 15 ;scans all lines in the 16x16 pattern pattern(x,y)= pattern(x,y) +Sin(x/8*Fx*Pi+px)* Sin(y/8*Fy*Pi+py)* ax* ay ; Synthesizes noise patternby additive mixing of all valid 2D frequencies Next y Next x End If NextFy Next Fx

APPENDIX E *** BASIC Program to convert unequally spread floating pointnumbers to integers *** For Value = 0 To 255 ; scans all 8-bit integervalues to obtain Min = 1000000 ; initializes the minimum value as “largeenough” For x = 0 To 15 ; scans all columns in the 16x16 pattern For y =0 To 15 ; scans all lines in the 16x16 pattern If pattern(x, y) < MinThen ; compares current value with minimum Min = pattern(x, y) ; tracksthe minimum floating point (FP) value min_x = x ; tracks the columnlocation of the minimum value min_y = y ; tracks the line location ofthe minimum value End If Next y Next x pattern(min_x, min_y) = Value ;replaces minimum FP value by the desired integer Next Value

APPENDIX F *** Program to diffuse peak values based on additivesynthesized pattern *** For x = 0 To 15 ; scans all columns in the 16x16pattern For y = 0 To 15 ; scans all lines in the 16x16 pattern DTNUL(x,y) = 100 ; initializes distances to nearest used location If pattern(x,y) < 128 Then ; separates lower 128 values from upper 128 valuespattern(x, y) = pattern(x, y) − 10000 ; shifts values to large negativeElse pattern(x, y) = pattern(x, y) + 10000 ; shifts values to largepositive End If Next y Next x LDTAUL = 100 ; initializes largestdistance to any used location For Value = 0 To 127 ; scans the lowerhalf of 8-bit integer values to obtain Min = −1 ; initializes theminimum value For x = 0 To 15 ; scans all columns in the 16x16 patternFor y = 0 To 15 ; scans all lines in the 16x16 pattern If DTNUL(x, y) >=LDTAUL * threshold Then ; marks a location as valid If Min > pattern(x,y) Then ; tracks the minimum value found in the valid area Min =pattern(x, y) xc = x ; tracks the best location column in the valid areayc = y ; tracks the best location line in the valid area End If End IfNext y Next x pattern(xc, yc) = Value ; assigns currently scanned valueto best found location LDTAUL = 0 ; initializes largest distance to anyused location For x = 0 To 15 ; scans all columns in the 16x16 patternFor y = 0 To 15 ; scans all lines in the 16x16 pattern distance = Sqr((8− Abs(8 − Abs(x − xc))) {circumflex over ( )} 2 + (8 − Abs(8 − Abs(y −yc))) {circumflex over ( )} 2) ; calculates the so called pan distanceIf DTNUL(x, y) > distance Then DTNUL(x, y) = distance ; tracks thedistance to the nearest used location If (pattern(x, y)<0) and(LDTAUL<DTNUL(x, y)) Then LDTAUL = DTNUL(x, y) Next y ; tracks theLDTAUL value when location is valid Next x Next Value For x = 0 To 15 ;scans all columns in the 16x16 pattern For y = 0 To 15 ; scans all linesin the 16x16 pattern DTNUL(x, y)= 100 ; reinitializes distances tonearest used location Next y Next x LDTAUL = 100 ; initializes largestdistance to any used location For Value = 255 To 128 step −1 ; scans theupper half of 8-bit integer values to obtain Max = 256 ; initializes themaximum value For x = 0 To 15 ; scans all columns in the 16x16 patternFor y = 0 To 15 ; scans all lines in the 16x16 pattern  If DTNUL(x,y) >= LDTAUL * threshold Then ; marks a location as valid If Max <pattern(x, y) Then ; tracks the maximum value found in the valid areaMax = pattern(x, y) xc = x ; tracks the best location column in thevalid area yc = y ; tracks the best location line in the valid area EndIf End If Next y Next x pattern(xc, yc) = Value ; assigns currentlyscanned value to best found location LDTAUL = 0 ; initializes largestdistance to any used location For x = 0 To 15 ; scans all columns in the16x16 pattern For y = 0 To 15 ; scans all lines in the 16x16 patterndistance = Sqr((8 − Abs(8 − Abs(x − xc))) {circumflex over ( )} 2 + (8 −Abs(8 − Abs(y − yc))) {circumflex over ( )} 2) ; calculates the socalled pan distance If DTNUL(x, y) > distance Then DTNUL(x, y) =distance ; tracks the distance to the nearest used location If(pattern(x, y)>256) and (LDTAUL<DTNUL(x,y)) Then LDTAUL=DTNUL(x, y) ;tracks the LDTAUL value when location is valid Next y Next x Next Value

APPENDIX G *** BASIC Code to synthesize blue noise spectrum withoutspecified pattern energy peaks *** golden_ratio = (Sqr(5) − 1) / 2 ;defines the golden ratio number For x = 0 To 15 ; scans all columns inthe 16x16 pattern For y = 0 To 15 ; scans all lines in the 16x16 patternpattern(x, y) = −10000 ; initializes the 16x16 noise pattern values ;the large negative offset simplifies further  computations Next y Next xFor Fx = 1 To 8 ; scans all horizontal frequencies Fx except 0 For Fy =1 To 8 ; scans all horizontal frequencies Fy except 0 If (Fx {circumflexover ( )} 2 + Fy {circumflex over ( )} 2) > (Poisson_Radius * 8){circumflex over ( )} 2 Then ; 2D-frequency must be above specifiedfraction of maximum frequency px = Rnd(1) * 2 * Pi ; generates a randomhorizontal phase py = Rnd(1) * 2 * Pi ; generates a random verticalphase If Fx = 8 Then ax = golden_ratio Else ax = 1 ; defines horizontalattenuation ax If Fy = 8 Then ay = golden_ratio Else ay = 1 ; definesvertical attenuation ax If Fx= 8 and Fy= 8 Then ax= 0 ; Completelysuppresses Peak(1) in FIG. 21 If Fx= 8 and Fy= 0 Then ax= ax/2 ;Attenuates Peak(2) according to FIG. 21 If Fx= 4 and Fy= 4 Then ax=ax*7/8 ; Attenuates Peak(3) with energy = 0125 in FIG. 21 If Fx= 8 andFy= 4 Then ax= ax*0.912 ; Attenuates Peak(4) with energy{circumflex over( )} 0.088 in FIG. 21 For x = 0 To 15 ; scans all columns in the 16x16pattern For y = 0 To 15 ; scans all lines in the 16x16 patternpattern(x,y)= pattern(x,y) + Sin(x/8*Fx*Pi+px)* Sin(y/8*Fy*Pi+py)* ax*ay ; Synthesizes noise pattern by additive mixing of all valid 2Dfrequencies Next y Next x End If Next Fy Next Fx

The invention claimed is:
 1. A method of processing a digital image signal in a processing chain between an image generator and an image display, the digital image signal being a video signal, wherein a dither pattern is applied to the image signal at a point in the processing chain, the dither pattern has a first energy spectrum, the method comprising the steps: applying a noise pattern to the image signal in the processing chain, the noise pattern having a second energy spectrum, which is configured such that the combination of the first energy spectrum of the dither pattern and the second energy spectrum of the noise pattern results in a more continuous energy spectrum, wherein the first energy spectrum of the dither pattern has a set of peaks and the second energy spectrum of the noise pattern has minima at positions corresponding to at least some of the peaks in the first energy spectrum, and wherein the first and second energy spectrums are energy spectrums that are functions of frequency.
 2. The method according to claim 1, wherein the dither pattern and the noise pattern are applied to the image signal one after the other in any order.
 3. The method according to claim 1, wherein the noise pattern has a coloured noise spectrum.
 4. The method according to claim 3, wherein the noise pattern has a blue noise spectrum.
 5. The method according to claim 1, wherein the dither pattern is an array of offset values which is tiled across the image.
 6. The method according claim 1, wherein the noise pattern is an array of offset values which is tiled across the image.
 7. The method according to claim 5, wherein the dither pattern is an A×B array of offset values which is tiled across the image and wherein the noise pattern is an N×M array of offset values which is tiled across the image, where N×M>A×B.
 8. The method according to claim 1, wherein the noise pattern has an amplitude response which is at least as large as an amplitude response of the dither pattern.
 9. The method according to claim 1, wherein the image signal is a frame of a video signal and the noise pattern is varied temporally.
 10. The method according to claim 9, wherein the noise pattern is modulated over a plurality of frames of the video signal.
 11. The method according to claim 9, wherein the noise pattern comprises a set of different stored noise patterns which are sequentially applied to frames of the video signal.
 12. The method according to claim 1, wherein the image signal contains different colour components and wherein there is a different noise pattern for each colour component of the image signal.
 13. The method according to claim 1, further comprising storing the noise pattern as an array in a look-up table and accessing elements of the array as a function of the line and row number of the image signal.
 14. The method according to claim 1, wherein the dither pattern is used in the processing chain prior to a reduction in colour bit depth.
 15. The method according to claim 1, wherein the noise pattern is applied before the dither pattern in the processing chain.
 16. The method according to claim 1, wherein the noise pattern is applied after the dither pattern in the processing chain.
 17. The method according to claim 1, wherein the noise pattern comprises an array of values, the values of the array being arranged in columns and lines and the array of values having a range of values and a spectral energy distribution, wherein the array of values have at least one of the following properties: the values in the array are linearly distributed across the range of values, and each value in the range of values occurs a substantially equal number of times within the array; values close to or at extreme ends of the range of values are dispersed from each other within the array; values are positioned in the array based on distance to values that are close in value to the respective values in neighbouring repetitions of the array; the spectral energy distribution is a substantially Poisson-disk two-dimensional spectral energy distribution; a sum of said values in any column of the array is substantially equal to a sum of said values in any other column of the array; a sum of said values in any line of the array is substantially equal to a sum of said values in any other line of the array.
 18. The method according to claim 1, wherein the second spectrum of the noise pattern is created by additive frequency synthesis.
 19. The method according to claim 1, further comprising detecting when applying the noise pattern to the image signal causes clipping to occur and suppressing the clipping.
 20. A digital image processing device comprising a processing block of a digital circuit for applying a noise pattern to a digital image using the method according to claim
 1. 21. A noise pattern generator for use in the method of claim
 1. 22. A non-transitory machine readable data storage device storing a computer program configured to perform the method of processing a digital image signal according to claim 1 when executed by a processor on a computer.
 23. A video processing device comprising a processing block of a digital circuit for applying a noise pattern to a video signal using the method according to claim
 1. 24. The method according to claim 1, wherein the second spectrum is substantially linear with the minima interrupting the linearity of the second spectrum.
 25. A method of generating a noise pattern for use in a digital image processing chain between an image generator and an image display, where a dither pattern is applied to the image signal at a point in the processing chain, the image signal being a video signal, the dithering signal having a first energy spectrum, the method comprising the steps: generating via a processor, the noise pattern with a second energy spectrum, which is configured such that the combination of the first energy spectrum and second energy spectrum results in a more continuous energy spectrum, wherein the first energy spectrum has a set of peaks and the second energy spectrum has minima at positions corresponding to at least some of the peaks in the first energy spectrum, and wherein the first and second energy spectrums are energy spectrums that are functions of frequency.
 26. The method according to claim 25, wherein the dither pattern and the noise pattern are applied to the image signal one after the other in any order.
 27. A digital image processing device comprising: a processing block for applying a noise pattern to a digital video signal, wherein a dither pattern is applied to the video signal at a point in the processing chain, the dither pattern having a first energy spectrum, wherein the processing block is configured to apply a noise pattern to the video signal in the processing chain, the noise pattern having a second energy spectrum which is configured such that the combination of the first energy spectrum and second energy spectrum results in a more continuous energy spectrum, wherein the first energy spectrum of the dither pattern has a set of peaks and the second energy spectrum of the noise pattern has minima at positions corresponding to at least some of the peaks in the first energy spectrum, and wherein the first and second energy spectrums are energy spectrums that are functions of frequency.
 28. The digital image processing device according to claim 27, wherein the dither pattern and the noise pattern are applied to the video signal one after the other in any order. 